mounting guidescope on top of refractor, Losmandy G11

All,

I want to mount my 5" refractor on my G11, and then mount a guide
scope on top of the refractor. I currently use the side-saddle
system, and I don't like having to balance on two axes. I've spoken
to one vendor who has a solution, but I wanted to ask for input from
anyone who's done this successfully (or not!) Whatever I do needs to
be suitable for long exposure astrophotography.

TIA,

-Dick L.
www.dl-digital.com

On Sat, 28 Aug 2004 12:10:57 GMT, DL wrote:

I want to mount my 5" refractor on my G11, and then mount a guide
scope on top of the refractor. I currently use the side-saddle
system, and I don't like having to balance on two axes. I've spoken
to one vendor who has a solution, but I wanted to ask for input from
anyone who's done this successfully (or not!) Whatever I do needs to
be suitable for long exposure astrophotography.

I spent quite a while getting a guider system working solidly. Initially, I had
an ST80 guidescope mounted on a 12" LX200 using Guide Star adjustable rings.
These rings proved to be inadequate, with flexure that couldn't be eliminated- I
would definitely not recommend them. I switched to Losmandy dovetail plates and
rings, and that took care of flexure- they are very robust. I identified some
remaining image shift that was caused by the focuser and by the guide camera
rocking in the draw tube. The first was fixed with an extra lock screw, and the
second by shimming with Mylar tape.

The weakness in this system was that the FOV of the guidescope was still small
enough that I had to adjust its position to find guide stars. This was both
inconvenient and resulted in occasional image shift from the three-point screw
mount system. My current solution, which is completely hands-off, is a rigidly
mounted achromatic objective on one Losmandy plate, and a rigidly mounted camera
on another. I use a 200mm f/5.1 lens and an ST237A. The FOV is over 1 degree on
a side; the guider never needs to be aimed or focused. You can see this system
he http://www.cloudbait.com/observatory/guider.jpg (with the floating tube
between the lens and camera removed for clarity).

I highly recommend a guidescope system that gives you a large enough FOV to
eliminate the requirement for positioning. I guide at 7 arcsec/pixel, and image
at 0.8 arcsec/pixel, and my guiding errors are smaller than I can measure. It is
very desirable to have a short focal length guidescope.

In your case, I'd keep it simple. A Losmandy plate and rings for the refractor,
and a rigidly mounted guider on separate Losmandy hardware on the that. For less
flexure, you might even try a custom solution. Make a new riser block for the
forward ring with a hole through it for the guider objective. Now you can place
a rigidly mounted guider camera underneath the piggybacked refractor.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris, thanks for the reply. I didn't mention that I use a ST80 with an STV
auto guider. I use the focal reducer on the STV's head, and have an effective
field equivalent to a 240mm lens with that setup. Pretty wide, though
occasionally I have trouble finding a guide star as well.

I went through all the same guiding issues myself. I think my biggest issues
now are with getting mount balanced & getting the polar alignment exactly right.

-Dick L.

On Sat, 28 Aug 2004 14:15:47 GMT, Chris L Peterson
wrote:

On Sat, 28 Aug 2004 12:10:57 GMT, DL wrote:

I want to mount my 5" refractor on my G11, and then mount a guide
scope on top of the refractor. I currently use the side-saddle
system, and I don't like having to balance on two axes. I've spoken
to one vendor who has a solution, but I wanted to ask for input from
anyone who's done this successfully (or not!) Whatever I do needs to
be suitable for long exposure astrophotography.

I spent quite a while getting a guider system working solidly. Initially, I had
an ST80 guidescope mounted on a 12" LX200 using Guide Star adjustable rings.
These rings proved to be inadequate, with flexure that couldn't be eliminated- I
would definitely not recommend them. I switched to Losmandy dovetail plates and
rings, and that took care of flexure- they are very robust. I identified some
remaining image shift that was caused by the focuser and by the guide camera
rocking in the draw tube. The first was fixed with an extra lock screw, and the
second by shimming with Mylar tape.

The weakness in this system was that the FOV of the guidescope was still small
enough that I had to adjust its position to find guide stars. This was both
inconvenient and resulted in occasional image shift from the three-point screw
mount system. My current solution, which is completely hands-off, is a rigidly
mounted achromatic objective on one Losmandy plate, and a rigidly mounted camera
on another. I use a 200mm f/5.1 lens and an ST237A. The FOV is over 1 degree on
a side; the guider never needs to be aimed or focused. You can see this system
he http://www.cloudbait.com/observatory/guider.jpg (with the floating tube
between the lens and camera removed for clarity).

I highly recommend a guidescope system that gives you a large enough FOV to
eliminate the requirement for positioning. I guide at 7 arcsec/pixel, and image
at 0.8 arcsec/pixel, and my guiding errors are smaller than I can measure. It is
very desirable to have a short focal length guidescope.

In your case, I'd keep it simple. A Losmandy plate and rings for the refractor,
and a rigidly mounted guider on separate Losmandy hardware on the that. For less
flexure, you might even try a custom solution. Make a new riser block for the
forward ring with a hole through it for the guider objective. Now you can place
a rigidly mounted guider camera underneath the piggybacked refractor.

_______________________________________________ __

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"DL" wrote in message
...
All,

I want to mount my 5" refractor on my G11, and then mount a guide
scope on top of the refractor. I currently use the side-saddle
system, and I don't like having to balance on two axes. I've spoken
to one vendor who has a solution, but I wanted to ask for input from
anyone who's done this successfully (or not!) Whatever I do needs to
be suitable for long exposure astrophotography.

TIA,

-Dick L.
www.dl-digital.com
Seriously, the side saddle system, should be 'set and forget'. Once the
system is balanced, you should not need to change it again. There is
nothing to stop you piggybacking multiple scopes as you describe (a couple
of manufacturers do rings with plate attachment points both at the top and
the bottom), _but_ you will be increasing the lever arm that the outer
items have from the scope axis, requiring more counterweight, and
increasing the risk of vibration, and the loads on the mount....

Best Wishes

Yes but...

First, the side-saddle system is difficult to balance. I don't know of a good
way to do it, really. Maybe I need a rubber mallet. In particular, it's
difficult to balance the two scopes on the saddle plate. There's no good way
that I know of to slide the plate one way or the other, a very small distance is
a big deal.

Second, if I want to change from my guide scope to a camera/lens combination, I
need to rebalance.

One solution might be some sort of sliding weight, that would allow you adjust
the balance of the two scopes without actually sliding the plate...

-Dick L.
On Fri, 03 Sep 2004 15:23:31 GMT, "Roger Hamlett"
wrote:


"DL" wrote in message
.. .
All,

I want to mount my 5" refractor on my G11, and then mount a guide
scope on top of the refractor. I currently use the side-saddle
system, and I don't like having to balance on two axes. I've spoken
to one vendor who has a solution, but I wanted to ask for input from
anyone who's done this successfully (or not!) Whatever I do needs to
be suitable for long exposure astrophotography.

TIA,

-Dick L.
www.dl-digital.com
Seriously, the side saddle system, should be 'set and forget'. Once the
system is balanced, you should not need to change it again. There is
nothing to stop you piggybacking multiple scopes as you describe (a couple
of manufacturers do rings with plate attachment points both at the top and
the bottom), _but_ you will be increasing the lever arm that the outer
items have from the scope axis, requiring more counterweight, and
increasing the risk of vibration, and the loads on the mount....

Best Wishes